Hi.. stupid question.. how to lock this car 100%?

Situation..

Forgot about the key and left the keys at home.. go outside to eat @ kedai makan about 50km from my house..  Reached at the kedai makan and left my car unlocked but switch off the engine. After that my sister go to the car and ask for the key.. Notice thatthe key is at home (Should be a big problem).. But when she tried to start the engine, the engine start like she have the key!

Problem is, the keys left at the home, but why she still can start the engine?

Imagine that you left the car unlocked then someone get into the car...

Ps: Unlocked the car because i parked the car in front of kedai makan .. about 5 meter from me..

edited: found the problem, spare key lefted in the car, haha.. just bought the car, forgot to keep it! soooo sorry!